The synthetic production of liquid fuels, lubricants, alcohols, alcohol derivatives, and other oxygenates from natural gas or other sources of synthesis gas (syngas) is well known and commonly called a gas to liquid (GTL) process. Basically, a carbonaceous material is first converted into a gas comprising carbon monoxide and hydrogen, i.e., syngas. Then the syngas is catalytically converted into liquid products via the well known Fischer-Tropsch process.
Processes for gasifying carbonaceous material into syngas are known in the industry. Three basic methods have been heretofore employed. These are: (1) the steam reforming of one or more light hydrocarbons such as methane over a catalyst; (2) the sub-stoichiometric partial oxidation of one or more light hydrocarbons; and (3) the combination of partial oxidation and steam reforming known as autothermal reforming.
Autothermal reforming has the advantage that the heat generated by the partial oxidation process is used to supply the heat necessary for the endothermic steam reforming reaction. Additionally, the autothermal process results in a lower hydrogen to carbon monoxide ratio in the syngas than does steam reforming alone. It is known that the H2/CO stoichiometric ratio for the F-T process is approximately 2:1, but there are many reasons for using other than a stoichiometric ratio. Steam (H2O) and CO2 can be added to the feed of any of the three syngas processes to modify the product H2/CO ratio. Additional steam increases the H2/CO ratio, while additional CO2 reduces the ratio.
The reaction products obtained in an F-T process depend upon the reaction conditions and catalyst employed. For example, high reaction temperatures favor the formation of alcohols, alcohol derivatives like ethers, and other oxygenates. Low reaction temperatures favor the formation of paraffinic liquid hydrocarbons and waxes. In either case, the product stream typically also includes, unreacted syngas, water and other gases such as CO2 and N2. The hydrocarbons and water are separated leaving a tail gas typically containing unreacted syngas, CO2, N2, Ar, gaseous desired products and water vapor. The N2 and Ar are typically present in either the feed gas or the oxygen fed to the autothermal reformer. The tail gas may be used as a fuel for the GTL plant, or part of the tail gas may be used as a source of CO2 for the autothermal reformer. For either use, the composition of the tail gas is substantially less than optimal due to the presence of the N2 and Ar species which dilute the concentration of the reactants leading to larger equipment, and in the case of N2 may form undesirable HCN and NH3 by-products during reforming.